The present invention relates to a matrix display device comprising two groups of row electrodes and two column electrodes per image or picture element, as well as to its control process.
The invention is used in optoelectronics and mainly in the control of liquid crystal cells used more particularly as a convertor of electrical information into optical information, in the real time processing of optical images and for analog display purposes.
More specifically, the invention relates to a matrix display device using a display material having an optical property and incorporating a plurality of image elements controlled by switches, such as thin film transistors.
The optical property of the display material used is e.g. opacity, refractive index, transparency, diffusion, diffraction, convergence, etc. Moreover, said material can be solid, liquid, amorphous or crystalline.
FIG. 1 diagrammatically shows a known matrix display device of this type. FIG. 1 shows two insulating walls, namely a first wall 1 and a second wall 3 which face one another and which are kept spaced and sealed by a joint 5. Between said walls is placed a display material layer 7 having an optical property.
Over the inner face of one of the walls 1 are distributed n parallel row conductors, designated L.sub.i and m parallel column conductors designated K.sub.j, which cross the row conductors, i and j being integers such that 1.ltoreq.i.ltoreq.n and 1.ltoreq.j.ltoreq.m. The row and column conductors carry electrical signals appropriate for exciting material 7 and generated by not shown addressing means. At the intersection 11 of each row conductor L.sub.i and each column conductor K.sub.j is provided a switch R.sub.ij, such as a thin film transistor, connected to an electrode D.sub.ij.
Moreover, the inner face of the other wall 3 is covered with a conductive material serving as a counterelectrode 13, which is raised to a reference potential.
An image or picture element A.sub.ij is defined in said device by the overlap region of an electrode D.sub.ij and the counterelectrode 13, electrode D.sub.ij and counterelectrode 13 respectively forming the two coatings of a capacitor between which is placed the display material.
In the particular case where the display material is a liquid crystal film, in order to prevent deterioration thereof, the counterelectrode 13 is raised to a reference potential, whose value is periodically inverted and the row conductors and column conductors carry alternating voltages.
In order to select a particular image element A.sub.ij of the device, an electrical signal is supplied on row conductor L.sub.i, which selects the conductive state of the group of transistors connected to said row conductor and in particular the conductive state of transistor R.sub.ij associated with said image element. This transistor then transmits to the electrode D.sub.ij to which it is connected the electrical signal from column K.sub.j. Between electrode D.sub.ij and counterelectrode 13, there is an electrical field which will cause a collective orientation of the molecules, particularly the liquid crystal inserted between the capacitor coatings formed by electrode D.sub.ij and counterelectrode 13, when the signal from column K.sub.j is equal to or exceeds the threshold voltage corresponding to the minimum value necessary for exciting material 7. This collective orientation will modify the optical property from material 7 to image element A.sub.ij.
By utilizing the selective orientation of the molecules and the punctiform excitation of the display material, an image is made to appear on the complete display device by defining it point by point.
Such a display device makes it possible to address a very large number of image elements. However, such means have a large number of defective image elements either in the form of whole columns and/or rows of defective image elements, or isolated defective image elements.
The poor operation of a row and a column of image elements is due either to short-circuits between the row of conductors and column conductors which cross one another, each short-circuit putting out of order one row and one column of image elements, or to short-circuited transistors, each of which also puts out of order one row and one column of image elements. The unsatisfactory operation of an isolated image element is e.g. due to an insulation defect of the transistor corresponding to said element.
In order to overcome these shortcomings, numerous display devices have been constructed in which the number of row conductors and column conductors addressing an image element has been multiplied leading to an increase in the number of transistors associated with an image element. This solution suffers from numerous disadvantages, such as e.g. increase in the complexity and cost of the display means and without eliminating all the fault and error sources. Thus, if e.g. the column conductors of a double redundant pattern corresponding to the addressing of the same column of image elements are respectively associated with a defective transistor, all the column of image elements will be out of order. Moreover, such devices do not eliminate isolated defective image elements.
In addition, this solution requires the use of tests during the construction of the display device, in order to reveal the defective elements and disconnect them, which also increases the manufacturing costs of the display device and which constitutes a source of subsequent faults.
FR-A-2553 218 describes a matrix display device obviating the crossing or intersection of row conductors and column conductors on the inner face of one of the walls of the display device. For this purpose, on one of the walls of the display device are arranged parallel column conductors with which are associated electrodes disposed in accordance with a first matrix and on the other wall are disposed parallel row conductors connected by transistors to a second matrix of electrodes facing the first matrix of electrodes. An image element of this device is defined by the overlap zone of an electrode of the first matrix and an electrode of the second matrix.
This display device makes it possible to obviate short-circuits between a row conductor and a column conductor, but a short-circuited transistor can still put out of operation a complete row conductor and therefore a row of image elements. Moreover, this device does not make it possible to obviate isolated defective image elements.
French patent application FR-A-2581783 in the name of the same Applicant describes a matrix display device making it possible to avoid any intersection between row conductors and column conductors on the inner face of a wall of the display device and prevents a short-circuited transistor from putting out of operation a row or column of image elements.
On the inner face of one of the walls of said device are arranged n parallel row conductors, each row conductor being connected to m transistors, each connected to an electrode of a first family and to an electrode of a second family, said electrodes being juxtaposed with said row conductor. On the inner face of the other wall of said device are arranged a first family of m parallel column conductors and a second family of m parallel column conductors, the column conductors of the first family alternating with the column conductors of the second family, one of the two families of column conductors being raised to a reference potential. The column conductors of the first and second families intersect with the row conductors, each column conductor of the first family facing n electrodes of the first family and each column conductor of the second family faces n electrodes of the second family. An image element of said device is defined by the overlap zone of two column conductors respectively of the first and second families with two electrodes respectively of the first and second families connected by a transistor to a row conductor. However, such a device does not make it possible to obviate poor isolated image elements.
The problem of the present invention is therefore to obviate this disadvantage by realizing a device making it possible to avoid the deficiency of both lines or columns of image elements and isolated image elements, particularly by dividing each of the two electrodes and the transistor of an image element of the display device described in FR-A-2 581 783 into several small electrodes and as many small transistors.